The present invention relates to the conduction of deep cooled, e.g., cryogenic fluids under utilization of at least two concentrically arranged corrugated metal tubes spaced apart by suitable spacers, and wherein the annular space between the two metal tubes is evacuated while certain super-insulation is placed in that gap.
Cryogenic tubing is known, for example, for the conduction of liquid helium, wherein the conduit system is comprised of several concentrically arranged corrugated tubes, and wherein the innermost tube conducts the cryogenic liquid, i.e., helium, while the annular spacs between the particular inner tube and the respective next, radially outer tube is either used for a return flow or for purposes of thermal insulation. Generally speaking, the evacuation of annular space between two concentric tubes is very advantageous for obtaining a high degree of thermal insulation, and for reducing concurrently radial losses, it is advisable to provide a so-called super insulation in that space. This super insulation is comprised essentially of several layers, for example, of a synthetic ribbon coated with a metal layer, and the individual lays of this configuration may be kept in spaced relation by means of a synthetic mesh or the like.
The known conduit systems are disadvantaged by the fact that after the generation of a vacuum between the two metal tubes certain gases may emanate from the synthetic material which is used for some reason or another, for example, for purposes of super insulation in the annular gap space between the metal tube. The emanated gases therefore cause deterioration of the vacuum, and in fact the thermal insulation of this gap is drastically reduced. Another significant disadvantage of the known construction is to be seen in that the synthetic material used as insulation is not sufficiently resistant against higher temperature. For example, in the case of a temperature increase by about 100 degrees centigrade, these synthetics deform and thereby cause a change in the distance between the metal layers. These increased temperatures do not of course arise during normal operation, particularly during the conduction of a cryogenic liquid. However, it was observed that these high temperatures can in fact occur during the evacuation process of the gap space, i.e., prior to the use of the system. This is so because for reasons of efficiency evacuation is carried out usually at elevated temperatures, and temperatures in the range of 350 to 400 degrees centigrade for this purpose are quite common. The reason for heating the gas to be evacuated, particularly when the super insulation has already been included, is to be seen in that the surface of the super insulation can be depleted from gas molecules easier and faster when the temperature of the system as a whole is raised. This means that the period of time needed for evacuation, i.e., the time of pumping and the time generally of generating the vacuum, can be drastically reduced if one operates at an elevated temperature level as stated. This of course is seemingly an economical procedure. However, it was found that the temperature increase has detrimental effects of the type mentioned earlier.
A further disadvantage of the known method is to be seen in that amazingly enough the metal coated foils and synthetic lattice structure are not adequately resistant against radiation, but decompose under a certain radiation load. In view of the fact that the radiation is not a very strong one, this effect works slowly, but this is exactly a highly disadvantageous aspect because a seemingly perfect conduit system suddenly fails for reasons of the slowly working decomposing effect in its insulation structure. In fact it was found that the decomposition of the synthetic involved may produce direct contact between metal layers which were originally spaced apart metal coatings, and due to unfortunate circumstances several metal layers can come into contact and provide a heat conducting bridge in between an inner and an outer tube and in a completely inforeseeable manner, reducing locally the thermal insulation very drastically, and that in turn can lead to a complete system failure.